This project seeks to define the basis for anti-proliferative effect of DTP-derived agents found to be active in inhibiting the growth of prostate carcinoma explants. These compounds are potentially acting through novel cytotoxic or cytostatic mechanisms. In addition, this project seeks to define the basis for rational use of drugs in prostate carcinoma based on an understanding of the unique cellular milieu of the prostate carcinoma cell. The compounds studied during the project period include: brefeldin A, jasplakinolide, and cucurbitacins. The studies demonstrate that brefeldin A requires protracted periods of exposure to prostate carcinoma cells to effect persistent growth inhibition: between 24 and 72 hr of exposure to between 50 and 100 nM will result in growth inhibition for at least another 72 to 96 hr. Jasplakinolide, a cyclodepsipeptide from the sponge J. johnstoni inhibited prostate carcinoma explant growth with IC50s of about 1 - 2 nM. This drug caused striking alterations of the actin cytoskeleton, with little effect on the tubulin cytoskeleton or macromolecular synthesis. Jasplakinolide caused inappropriate cation- independent polymerization of purified actin. Cucurbitacins, which inhibited growth of prostate carcinoma explants with IC50s also approximately 10 - 20 nM provoked profound alterations of the actin cytoskeleton, although direct interaction with purified cytoskeletal components has not yet been demonstrated. These studies therefore indicate that the prostate explant screen detects compounds as potent growth inhibitors compounds which target structural elements which are not recognized as targets for conventional chemotherapeutic agents, including the Golgi apparatus and other endomembrane compartments in the case of brefeldin, and the actin cytoskeleton in the case of jasplakinolide and possibly cucurbitacins.